Provide completions for spaghetto

[deemp]

I used purescript-overlay and couldn't get completions for spaghetto 0.93.9 using nix run .#spago-unstable -- --bash-completion-script. It failed with this message

spago
    Unexpected argument:
        --bash-completion-script
...

Then, I searched the spago repo and found no completion entries in spaghetto.

It would be nice to have a documented option for spaghetto completions.

[f-f]

I think this should be raised upstream in the argparsing library that we use?

[deemp]

You can probably use https://github.com/f-o-a-m/purescript-optparse instead

[f-f]

I am not really up for rewriting the CLI handling myself, but I'd be willing to merge a PR that implements this and doesn't make it substantially worse to maintain

[deemp]

Closed by #980